(i) Technical Field
The present invention relates to an image forming system including a post-treatment apparatus that performs post-treatment to a sheet on which an image is formed, and a booklet finishing apparatus that is provided in the post-treatment apparatus and finishes a booklet produced by folding sheets.
(ii) Related Art
Conventionally, a post-treatment apparatus is added to some of image forming systems typified by a printer and a copying machine. The post-treatment apparatus produces the booklet as post-treatment of the sheets in which the images are formed. A method of folding the sheets to produce the booklet can be cited as an example of the booklet producing method. The booklet produced by the method of folding the sheets to produce the booklet has a curvature near a back. Therefore, in order to improve quality of the booklet, a square folding process is employed in which the back of the booklet is compressed to be flat.
FIG. 9 shows an outline of a square folding process.
Part (a) to part (d) of FIG. 9 sequentially show a state in which a square folding process is performed to a booklet 511. As shown in part (a) of FIG. 9, the booklet 511 is produced by folding the sheets. However, because the sheets are not completely folded by being folded only once, the booklet 511 has the curvature near a back 511a. As shown in part (b), the booklet 511 is conveyed while the back 511a is set at a front end and is positioned by causing the back 511a to abut on a stopper 514. A clamping unit 512 clamps the booklet 511 from both surfaces of the booklet 511 to flatten the booklet 511. Then, as shown in part (c), a roller 513 is pressed against the back of the booklet 511a. The roller 513 is moved along the back 511a while pressing the back 511a, and the roller 513 breaks down the back of the booklet 511a. Therefore, as shown in part (d), the back of the booklet 511a is flattened.
However, in the square folding process, sometimes the booklet quality is decreased depending on the kind of a booklet.
FIG. 10 shows a booklet different from the booklet 511 of FIG. 9.
FIG. 10 shows a state (a) in which the square folding process is performed and a state (b) in which the square folding process is not performed in a relatively thin booklet 521 different from the booklet 511 shown in FIG. 9. In the booklet 521 in the state (a) in which the square folding process is performed, the booklet 521 is relatively flattened compared with the booklet 521 in the state (b), but a projection portion 521b which is projected from a cover is formed in a part of the back. For the thin booklet, the square folding process is prohibited by selection of a user so as not to degrade the quality due to the projection portion 521b. As a result, although the projection portion 521b is not formed, the booklet is left unflattened, which is a problem in terms of finishing quality of the booklet.
Because the number of sheets processed per time is usually kept constant in the image forming apparatus, the number of booklets finished per time is increased as the number of sheets constituting the booklet is decreased. That is, in the case of a relatively thin booklet, a time interval necessary to produce a booklet is probably shorter than a time interval for one-time square folding process. In the case where the folded booklet is not bound, the time interval to produce the booklet is further shortened by the time interval to bind the booklet. In such cases, due to a processing time of the square folding process, a time waiting for the finish of the square folding process is generated on the side of the image forming apparatus or booklet producing unit, which decreases throughput of the whole.